package com.example.kaoshi;

import java.util.Scanner;

/**
 * Created by Quincy on 2018/9/29.
 */
public class Test22 {

    public static void main(String[] args){
        Scanner scanner = new Scanner(System.in);

        int n = scanner.nextInt();
        int[] arr = new int[n];
        for (int i =0; i < n; i++){
            arr[i] = scanner.nextInt();

        }
        findUniqueMember(arr);
    }
    public static void findUniqueMember (int[] array){
//        ArrayList<Integer> list = new ArrayList<>();
//        for (int i = 0 ; i < array.length; i++){
//            int e = array[i];
//            for (int j  = i + 1 ;j < array.length; j++){
//                if (array[j] != e){
//                    list.add()
//                }
//            }
//        }


        String arrayString = "";

        for (int i=0; i<array.length; i++){
            arrayString += array[i] + " ";
        }


        for (int j=0; j<array.length; j++){
            String tempString = arrayString.replaceAll(array[j]+"", " ").substring(0,array.length-1);

            if ((arrayString.length()-tempString.length()) == 1){
                System.out.println (array[j] );
                return;
            }
        }
        System.out.println("No");
    }
}
